There are two types of engine braking: the compression release braking and the bleeder braking. In compression release engine braking, a cam drives an engine exhaust valve to open the valve near the end of the compression stroke of the engine to release the compressed air. The energy absorbed by the compressed air during the compression stroke cannot be returned to the engine piston at the subsequent expansion or “power” stroke of the engine, but is dissipated by the engine exhaust and cooling systems, which results in an effective engine braking and the slow-down of the vehicle.
In addition to the normal exhaust valve opening during the exhaust stroke of the engine cycle, a bleeder brake opens an exhaust valve with a constant and small lift during all the remaining engine cycle (the intake, the compression and the expansion strokes) (full cycle bleeder brake) or during part of the remaining cycle (partial cycle bleeder brake). A bleeder brake is normally not driven by a cam, and must be used with the combination of an exhaust back pressure control device, such as an exhaust butterfly valve. But a compression release brake can be used alone (with no need of an exhaust back pressure control device).
U.S. Pat. No. 7,013,867 discloses a partial cycle bleeder brake system where the outer exhaust valve (away from the cam) is floated open with a small lift near the end of the engine intake stroke by increasing of the exhaust back pressure. The opened exhaust valve is then kept open by a hydraulic piston in the valve bridge with the small lift during the whole compression stroke, and finally closed in the early expansion stroke of the engine. China Patent 201110047127.5 (2014) improved this partial cycle bleeder brake system by integrating the braking bracket into the engine's exhaust rocker arm, reducing space and cost. Still, it is a bleeder brake that has lower engine braking power than a compression release brake, especially at middle to low engine speeds when the exhaust back pressure is low and the floating of the exhaust valve is small or none, which causes low braking or no braking at all. In addition, the valve floating from increasing exhaust back pressure by a butterfly valve or other exhaust flow restriction device could be excess, which leads to uncontrollable seating velocity of the valve and may cause engine damage.